1. Field of the Invention
The present invention relates to an optical film for reducing color shift and a liquid crystal display (LCD) having the same, and more particularly, to an optical film for reducing color shift and an LCD having the same, in which engraved or embossed lens sections are formed to reduce color shift depending on the viewing angle.
2. Description of Related Art
In response to the emergence of the advanced information society, components and devices related to image displays have been significantly improved and rapidly disseminated. Among them, image display devices have been widely distributed for use in TVs, personal computer (PC) monitors, and the like. Moreover, attempts are underway to simultaneously increase the size and reduce the thickness of such display devices.
In general, a liquid crystal display (LCD) is one type of flat panel display, and displays images using liquid crystals. The LCD is widely used throughout industry since it has the advantages of light weight, low drive voltage and low power consumption compared to other display devices.
FIG. 1 is a conceptual view schematically showing the basic structure and operating principle of an LCD 100.
With reference by way of example to a conventional vertical alignment (VA) LCD, two polarizer films 110 and 120 are arranged such that their optical axes are oriented perpendicular to each other. Liquid crystal molecules 150 having birefringence characteristics are interposed and arranged between two transparent substrates 130, which are coated with transparent electrodes 140. When an electric field is applied from a power supply unit 180, the liquid crystal molecules move and are aligned perpendicular to the electric field.
Light emitted from a backlight unit is linearly polarized after passing through the first polarizer film 120. As shown in the left of FIG. 1, the liquid crystal molecules remain perpendicular to the substrates when no power is applied. As a result, light that is in a linearly polarized state is blocked by the second polarizer film 110, the optical axis of which is perpendicular to that of the first polarizer film 120.
In the meantime, as shown in the right of FIG. 1, when power is on, the electric field causes the liquid crystal molecules to become horizontally aligned such that they are parallel to the substrates, between the two orthogonal polarizer films 110 and 120. Thus, the linearly polarized light from the first polarizer film is converted into another kind of linearly polarized light, the polarization of which is rotated by 90°, circularly polarized light, or elliptically polarized light while passing through the liquid crystal molecules before it reaches the second polarizer film. The converted light is then able to pass through the second polarizer film. It is possible to gradually change the orientation of the liquid crystal from the vertical orientation to the horizontal orientation by adjusting the intensity of the electric field, thereby allowing control of the intensity of light emission.
FIG. 2 is a conceptual view showing the orientation and optical transmissivity of liquid crystals depending on the viewing angle.
When liquid crystal molecules are aligned in a predetermined direction within a pixel 220, the orientation of the liquid crystal molecules varies depending on the viewing angle.
When viewed from the front left (210), the liquid crystal molecules look as if they are substantially aligned along the horizontal orientation 212, and the screen is relatively bright. When viewed from the front along the line 230, the liquid crystal molecules are seen to be aligned along the orientation 232, which is the same as the orientation inside the pixel 220. In addition, when viewed from the front left (250), the liquid crystal molecules look as if they are substantially aligned along the vertical orientation 252, and the screen is somewhat darker.
Accordingly, the viewing angle of the LCD is greatly limited compared to other displays, which intrinsically emit light, since the intensity and color of light of the LCD varies depending on changes in the viewing angle. A large amount of research has been carried out with the aim of increasing the viewing angle.
FIG. 3 is a conceptual view showing a conventional attempt to reduce variation in the contrast ratio and color shift depending on the viewing angle.
Referring to FIG. 3, a pixel is divided into two pixel parts, that is, first and second pixel parts 320 and 340, in which the orientations of liquid crystals are symmetrical to each other. Either the liquid crystals oriented as shown in the first pixel part 320 or the liquid crystals oriented as shown in the second pixel part 340 can be seen, depending on the viewing direction of a viewer. The intensity of light reaching the viewer is the total intensity of light of the two pixel parts.
When viewed from the front left (310), liquid crystal molecules in the first pixel part 320 look as if they are aligned along the horizontal orientation 312, and liquid crystal molecules in the second pixel part 320 look as if they are aligned along the vertical orientation 314. Thus, the first pixel part 320 makes the screen look bright. Likewise, when viewed from the front right (350), the liquid crystal molecules in the first pixel part 320 look as if they are aligned along the vertical orientation 352, and the liquid crystal molecules in the second pixel part 340 look as if they are aligned along the horizontal orientation 354. Then, the second pixel part 340 can make the screen look bright. In addition, when viewed from the front, the liquid crystal molecules are seen to be aligned along the orientations 332 and 334, which are the same as the orientations inside the pixel parts 320 and 340. Accordingly, the brightness of the screen observed by the viewer remains the same or similar, and is symmetrical about the vertical center line of the screen, even when the viewing angle changes. This, as a result, makes it possible to reduce variation in the contrast ratio and color shift depending on the viewing angle.
FIG. 4 is a conceptual view showing another conventional approach for reducing variation in the contrast ratio and color shift depending on to the viewing angle.
Referring to FIG. 4, an optical film 420 having birefringence characteristics is added. The birefringence characteristics of the optical film 420 are the same as those of liquid crystal molecules inside a pixel 440 of an LCD panel, and are symmetrical with the orientation of the liquid crystal molecules. Due to the orientation of the liquid crystal molecules inside the pixel 440 and the birefringence characteristics of the optical film, the intensity of light reaching the viewer is the total intensity of light from the optical film 420 and the pixel 440.
Specifically, when viewed from the front left (410), the liquid crystal molecules inside the pixel 440 look as if they are aligned along the horizontal orientation 414, and the imaginary liquid crystals produced by the optical film 420 look as if they are aligned along the vertical orientation 412. The resultant intensity of light is the total intensity of light from the optical film 420 and the pixel 440. Likewise, when viewed from the front right (450), the liquid crystal molecules inside the pixel 440 look as if they are aligned along the vertical orientation 454 and the imaginary liquid crystals produced by the optical film 420 look as if they are aligned along the horizontal orientation 452. The resultant intensity of light is the total intensity of light from the optical film 420 and the pixel 440. In addition, when viewed from the front, the liquid crystal molecules are seen to be aligned along the orientations 434 and 432, which are the same as the orientation inside the pixel 440 and the double-refracted orientation of the optical film 420, respectively.
However, even if the approaches described above are applied, there remains the problem shown in FIG. 5. That is, color shift still occurs depending on the viewing angle, and the color changes when the viewing angle increases.
In addition, optical films and display devices of the related art, in particular, TN mode LCDs, have the problem of gamma-curve distortion and grayscale inversion.
The information disclosed in this Background of the Invention section is only for the enhancement of understanding of the background of the invention, and should not be taken as an acknowledgment or any form of suggestion that this information forms a prior art that would already be known to a person skilled in the art.